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Abstract

The particulate optical backscattering coefficient (bbp) is a fundamental optical property that allows monitoring of marine suspended particles both in situ and from space. Backscattering measurements in the open ocean are still scarce, however, especially in oligotrophic regions. Consequently, uncertainties remain in bbp parameterizations as well as in satellite estimates of bbp. In an effort to reduce these uncertainties, we present and analyze a dataset collected in surface waters during the 19th Atlantic Meridional Transect. Results show that the relationship between particulate beam-attenuation coefficient (cp) and chlorophyll-a concentration was consistent with published bio-optical models. In contrast, the particulate backscattering per unit of chlorophyll-a and per unit of cp were higher than in previous studies employing the same sampling methodology. These anomalies could be due to a bias smaller than the current uncertainties in bbp. If that was the case, then the AMT19 dataset would confirm that bbp:cp is remarkably constant over the surface open ocean. A second-order decoupling between bbp and cp was, however, evident in the spectral slopes of these coefficients, as well as during diel cycles. Overall, these results emphasize the current difficulties in obtaining accurate bbp measurements in the oligotrophic ocean and suggest that, to first order, bbp and cp are coupled in the surface open ocean, but they are also affected by other geographical and temporal variations.

R. J. Geider, “Light and temperature dependence of the carbon to chlorophyll a ratio in microalgae and cyanobacteria: implications for physiology and growth of phytoplankton,” New Phytol. 106, 1–34 (1987).
[Crossref]

1987 (2)

R. J. Geider, “Light and temperature dependence of the carbon to chlorophyll a ratio in microalgae and cyanobacteria: implications for physiology and growth of phytoplankton,” New Phytol. 106, 1–34 (1987).
[Crossref]

R. J. Geider, “Light and temperature dependence of the carbon to chlorophyll a ratio in microalgae and cyanobacteria: implications for physiology and growth of phytoplankton,” New Phytol. 106, 1–34 (1987).
[Crossref]

Nature (1)

New Phytol. (1)

R. J. Geider, “Light and temperature dependence of the carbon to chlorophyll a ratio in microalgae and cyanobacteria: implications for physiology and growth of phytoplankton,” New Phytol. 106, 1–34 (1987).
[Crossref]

Comparison between Chl estimated from the AC9 and ACs instruments and HPLC derived TChl. The dashed line is the 1:1 line. N = 109. δr and σr are robust estimates of relative bias (median of the relative residuals) and precision (σ68 of the relative residuals), respectively.

Comparison between bbp measurements collected at two different wavelengths by the instruments mounted on the flow-through and profiling systems. Error bars represent the combined uncertainties in the bbp estimates. Dashed lines are the 1:1 lines. Note the logarithmic scales on all axes. δ and δr are robust estimates of the absolute and relative bias, respectively. σ and σr are estimates (σ68 of residuals) of the absolute and relative precision, respectively. N = 24.

Bivariate histograms showing the relationships between bbp(526) and bp(526) versus Chl. Solid and dashed lines are the models presented in refs. [12] and [10], respectively. The inset presents the bbp(526) vs. Chl bivariate histogram, but in linear scale (bbp is multiplied by 1000). The colorbar identifies the number of data points in each bivariate bin.

Normalized frequency distribution of bbp:cp measured during AMT19 (solid line) and of (bbp − bb02):cp (dashed line). All measurements are at 526 nm. The shaded area represents the mean value of the normalized frequency distributions of the datasets presented in ref. [8].

Time series of (a)bb02 with error bars indicating 95% confidence intervals and (b)bb02:bbp with error bars representing standard errors plotted at discrete locations for clarity. Black dashed and red solid vertical lines indicate times when the flow-through system was cleaned and when the 0.2μm filter was replaced, respectively.

Subset of time series showing diel cycles in each variable. Filled circles in the bottom plot are TChl-a estimates from discrete HPLC measurements. Values for γbbp were median filtered (window size of 120 minutes) to remove noise.

Tables (4)

Table 1 Scaling factors for the BB349 and BB499 meters. “Final values” are those used to process the data presented in this study. Absolute uncertainties are standard error of the means of the AMT19 determinations. All values, except relative uncertainties, are in units of sr−1 counts−1 × 10−6. Number in parentheses are percent changes from the WetLabs (07/2009) calibration.

Table 3 Uncertainties used to compute the combined experimental uncertainty of bbp as a function of wavelength. Units of absolute uncertainties are the same as those reported in the text for the corresponding variables.

Table 4 Uncertainty budget for bbp based on the values presented in Table 3 and on all backscattering measurements collected during the cruise in flow-through mode (BB349). Numbers represent the median values of the squared percent contributions,
σrel2 (unitless), and the median absolute contributions, σ (m−1), by each input variable to the combined experimental uncertainty in bbp as a function of wavelength.

Metrics

Table 1

Scaling factors for the BB349 and BB499 meters. “Final values” are those used to process the data presented in this study. Absolute uncertainties are standard error of the means of the AMT19 determinations. All values, except relative uncertainties, are in units of sr−1 counts−1 × 10−6. Number in parentheses are percent changes from the WetLabs (07/2009) calibration.

BB349

BB499

470

526

656

470

526

595

WetLabs (07/2009)

6.641

4.349

2.168

6.947

4.396

2.660

AMT19 (20/10/2009)

6.432(−3.2)

4.496(+3.4)

2.323 (+7.1)

7.347(+5.8)

4.975(+13.2)

-

AMT19 (22/11/2009)

6.771 (+2.0)

4.539(+4.4)

2.624(+21.0)

7.216(+3.9)

4.590 (+4.4)

2.535(−4.7)

Final values

6.601

4.517

2.473

7.281

4.782

2.598

Abs. uncertainties

0.170

0.021

0.151

0.066

0.192

0.063

Rel. uncertainties

0.026

0.005

0.061

0.009

0.040

0.024

Table 2

Dark counts provided by manufacturer and measured during AMT19. Uncertainties represent half the central 68th percentile of the measurements conducted during AMT19. All units are counts.

BB349

BB499

470

526

656

470

526

595

WetLabs

52

46

42

44

42

27

AMT19

56

51

50

53

48

50

AMT19 uncertainties

1.0

1.5

1.5

2.0

2.0

1.5

Table 3

Uncertainties used to compute the combined experimental uncertainty of bbp as a function of wavelength. Units of absolute uncertainties are the same as those reported in the text for the corresponding variables.

Table 4

Uncertainty budget for bbp based on the values presented in Table 3 and on all backscattering measurements collected during the cruise in flow-through mode (BB349). Numbers represent the median values of the squared percent contributions,
σrel2 (unitless), and the median absolute contributions, σ (m−1), by each input variable to the combined experimental uncertainty in bbp as a function of wavelength.

Variable

σrel2

σ × 10–4

470 nm

526 nm

470 nm

526 nm

χp

1

1

0.28

0.24

S

85

81

3.01

2.19

C

5

9

0.68

0.78

D

2

4

0.46

0.47

βsw

1

1

0.38

0.23

bb,wall

6

4

0.83

0.52

Tables (4)

Table 1

Scaling factors for the BB349 and BB499 meters. “Final values” are those used to process the data presented in this study. Absolute uncertainties are standard error of the means of the AMT19 determinations. All values, except relative uncertainties, are in units of sr−1 counts−1 × 10−6. Number in parentheses are percent changes from the WetLabs (07/2009) calibration.

BB349

BB499

470

526

656

470

526

595

WetLabs (07/2009)

6.641

4.349

2.168

6.947

4.396

2.660

AMT19 (20/10/2009)

6.432(−3.2)

4.496(+3.4)

2.323 (+7.1)

7.347(+5.8)

4.975(+13.2)

-

AMT19 (22/11/2009)

6.771 (+2.0)

4.539(+4.4)

2.624(+21.0)

7.216(+3.9)

4.590 (+4.4)

2.535(−4.7)

Final values

6.601

4.517

2.473

7.281

4.782

2.598

Abs. uncertainties

0.170

0.021

0.151

0.066

0.192

0.063

Rel. uncertainties

0.026

0.005

0.061

0.009

0.040

0.024

Table 2

Dark counts provided by manufacturer and measured during AMT19. Uncertainties represent half the central 68th percentile of the measurements conducted during AMT19. All units are counts.

BB349

BB499

470

526

656

470

526

595

WetLabs

52

46

42

44

42

27

AMT19

56

51

50

53

48

50

AMT19 uncertainties

1.0

1.5

1.5

2.0

2.0

1.5

Table 3

Uncertainties used to compute the combined experimental uncertainty of bbp as a function of wavelength. Units of absolute uncertainties are the same as those reported in the text for the corresponding variables.

Table 4

Uncertainty budget for bbp based on the values presented in Table 3 and on all backscattering measurements collected during the cruise in flow-through mode (BB349). Numbers represent the median values of the squared percent contributions,
σrel2 (unitless), and the median absolute contributions, σ (m−1), by each input variable to the combined experimental uncertainty in bbp as a function of wavelength.